A hot slab as thick as several hundreds of millimeters produced by continuous casting, etc. is rolled to a steel strip as thick as several millimeters to several tens of millimeters by a hot strip mill comprising roughing rolls and finishing rolls. A finishing mill usually comprises 5 to 7 four-high rolling stands arranged in tandem. In the case of a 7-stand finishing mill, first to third stands are called front stands, and fourth to seventh stands are called rear stands.
Because work rolls used in such a hot strip mill come into contact with a hot-rolled, thin strip, damages such as wear, surface roughening, heat cracking, etc. occur on their outer layer surfaces by a thermal and mechanical rolling load. After these damages are removed by grinding, the work rolls are used again for rolling. The grinding of a surface layer of a roll outer layer to remove damages is called “dressing.” After dressed from the initial diameter to the minimum diameter usable for rolling (discard diameter), the work roll is discarded. From the initial diameter to the discard diameter is called an effective rolling diameter. In the effective rolling diameter, an outer layer of a roll for hot rolling desirably has excellent wear resistance, sticking resistance and accident resistance, to prevent large surface damages such as heat cracking.
Dressing is classified to light dressing for removing surface damage due to usual rolling wear, and heavy dressing for removing surface damage due to rolling troubles. Particularly in rear finishing stands, a rolling trouble called “overlapped rolling,” in which folded or cut steel strips are rolled in an overlapped state, is likely to occur. When such trouble occurs, the roll surface is locally subject to strong pressure, so that a steel strip sticks to a roll surface. As a result, cracking is generated and propagates in the roll due to high heat and load. Particularly cracks generated by the rolling trouble tend to be extremely deep. Accordingly, rolls for hot rolling are required to suffer little wear by rolling (have excellent wear resistance), to be resistant to sticking (have excellent sticking resistance), and to be resistant to the propagation of cracking (have excellent accident resistance) even in rolling troubles.
Thus proposed as work rolls used in finishing rear stands in hot strip mills, which are required to have excellent wear resistance, sticking resistance and accident resistance, are composite rolls comprising outer layers made of alloys containing hard-carbide-forming elements such as Mo, V, etc. to provide high-alloy grain cast irons having good sticking resistance with improved wear resistance.
For example, JP 2005-105296 A discloses an outer layer of a roll for hot rolling, which has a composition comprising by mass 2.5-3.5% of C, 1.0-2.5% of Si, 0.3-1% of Mn, 3-5% of Ni, 1.5-2.5% of Cr, 1.0-4% of Mo, 1.4-3.0% of V, 0.1-0.5% of Nb, and 0.0005-0.2% of B, the balance being Fe and inevitable impurities, and a structure comprising 50000-1000000/mm2 of fine carbides having the maximum length of 0.1-5 μm in at least part of a matrix, thereby having excellent wear resistance and surface deterioration resistance. JP 2005-105296 A describes that the Ni grain roll is provided with improved wear resistance by MC carbides, with secondary carbides precipitated in a matrix to prevent surface roughening, and that for that structure, hardening is preferably conducted at 800-950° C. However, temperature difference occurs between the surface and inner portion of the roll in a cooling process from such hardening treatment, resulting in a residual compression stress applied to the roll surface. With this stress combined with a residual compression stress by the transformation expansion of the outer layer, the roll surface is subject to an extremely high residual compression stress. Thus, a high residual compression stress leads to cracking.
JP 2004-82209 A discloses a centrifugally cast hot-rolling composite roll comprising an outer layer having a chemical components comprising by mass 3.0-4.0% of C, 0.8-2.5% of Si, 0.2-1.2% of Mn, 3.0-5.0% of Ni, 0.5-2.5% of Cr, 0.1-3.0% of Mo, and 1.0-5.0% of V, the balance being Fe and inevitable impurities, and a shaft portion made of usual cast iron containing 2.5-4.0% of C or spheroidal graphite cast iron, the thickness T of the outer layer and the radius R of the shaft portion meeting the relation of 0.03≦T/R≦0.5. This composite roll has sticking resistance and wear resistance, free from breakage when produced, and spalling during use. However, with only a tempering treatment at 430° C. conducted as a heat treatment, the roll outer layer has insufficient hardness, and thus poor wear resistance.
JP 2002-88444 A discloses a composite roll comprising an outer layer made of a wear-resistant cast iron, an intermediate layer fused to an inner surface of the outer layer, and a shaft portion fused to an inner surface of the intermediate layer, the outer layer having a chemical composition comprising by weight 1.0-3.0% of C, 0.1-2.0% of Si, 0.1-2.0% of Mn, 0.1-4.5% of Ni, 3.0-10.0% of Cr, 0.1-9.0% of Mo, 1.5-10.0% of W, and 3.0-10.0% in total of V and/or Nb, the balance being substantially Fe; the intermediate layer having a chemical composition comprising by weight 1.0-2.5% of C, 0.2-3.0% of Si, 0.2-1.5% of Mn, 4.0% or less of Ni, 4.0% or less of Cr, 4.0% or less of Mo, 12% or less in total of W and/or V, and 12% or less in total of at least one of W, V and Nb, the balance being substantially Fe; and the shaft portion being made of flaky graphite cast iron, spheroidal graphite cast iron or graphite steel. However, because the outer layer contains as much as 3.0-10.0% of Cr, graphite is unlikely precipitated, resulting in poor sticking resistance and fracture toughness. Also, the precipitation of Cr carbides (M7C3, M23C6, etc.) provides the outer layer with low fracture toughness, by which the outer layer suffers more propagation of cracks by rolling troubles.
JP 09-170041 A discloses a centrifugally cast roll comprising a graphite-containing outer layer and a ductile cast iron shaft integrally fused via an intermediate layer of graphite steel, the outer layer comprising 2.5-4.7% of C, 0.8-3.2% of Si, 0.1-2.0% of Mn, 0.4-1.9% of Cr, 0.6-5.0% of Mo, 3.0-10.0% of V, and 0.6-7.0% of Nb, meeting the following formulae (1)-(4):2.0+0.15V+0.10Nb≦C (%)  (1),1.1≦Mo/Cr  (2),Nb/V≦0.8  (3), and0.2≦Nb/V  (4),the balance being Fe and inevitable impurities; the shaft comprising 2.8-3.8% of C, 2.0-3.0% of Si, 0.3-1.0% of Mn, 0.10% or less of P, 0.04% or less of S, 0.3-2.0% of Ni, 1.5% or less of Cr, and 1.0% or less of Mo, the balance being Fe and inevitable impurities; and the intermediate layer comprising 1.0-2.0% of C, 1.6-2.4% of Si, 0.2-1.0% of Mn, 0.05% or less of P, 0.03% or less of S, 0.1-3.5% of Ni, 1.5% or less of Cr, and 0.1-0.8% of Mo, the balance being Fe and inevitable impurities. However, when the intermediate layer is made of graphite steel, the intermediate layer has higher solidification start temperature than that of the outer layer, so that casting defects such as shrinkage cavities, etc. likely occur in the outer layer or the intermediate layer.